This invention relates to compact heat exchangers, particularly those in which the working fluid undergoes a change of state from liquid to vapor. In such exchangers a relatively high pressure drop across the exchanger on the working fluid side is not considered to be detrimental, because ample pressure energy is normally available in the liquid being vaporized to drive it through the exchanger, and a considerable pressure drop is desired in any event to utilize part of the energy stored in the liquid to supply part of the energy of vaporization.
Typical applications for such heat exchangers are as evaporators and vaporizers for liquefied petroleum, both in engine fuel systems and in gas plants. (The present invention will be discussed primarily in the context of motor fuel system type applications, although its advantages are also fully available in other applications.)
Heretofore, compactness was not a prime consideration in the design of vaporizers for liquefied petroleum for use in automotive type engines. The other parts of the fuel system were also fairly bulky, and the vaporizer, being conventionally associated with one or another of them, was customarily and conveniently made about the same size as its associated part. Thus, a fuel system with a five-inch diaphragm in the pressure regulator would likely have a five-inch diameter vaporizer attached to the regulator.
Until recently, the overall size of an engine fuel system was limited to fairly large proportions because of the use of a fixed venturi system to respond to the widely varying mass air flow demands of the engine. Recent developments in variable orifice and variable venturi techniques have made it possible to reduce the size of the fuel-air control portion of the system, and to make commensurate reductions in the size of the fuel control portion. This has led to a pressing need for compact vaporizers for such systems.
A small fuel supply system is, of course, desirable because of the crowded condition of engine compartments generally, and of automotive underhood engine compartments in particular.
Most vaporizers employed for liquefied petroleum fuel systems in the past have fallen into three classes: (a) bellows type; (b) labyrinth type; or (c) tubular type. In the bellows type liquid fuel is sprayed through a nozzle to impinge it and vapor being formed onto the internal heated walls of a bellows with a swirling action. See U.S. Pat. No. 3,176,709. Labyrinth heat exchangers are most common in larger fuel systems, where they are usually positioned between the primary and secondary pressure regulators. This type of vaporizer, as its name connotes, is usually an aluminum casting with curving intertwined grooves for the fuel and heating liquid, and an unheated metal lid closing the grooves. (See U.S. Pat. Nos. 3,184,295, and 2,832,204.) The unheated fourth side of the passages (formed by the lid) presents a condensation problem in this type of vaporizer. Tubular type vaporizers are also commonly inserted between the primary and secondary pressure regulators, although some units have loops or coils upstream from the primary regulator. This type of vaporizer has better thermal efficiency than the labyrinth type, but is more difficult and expensive to manufacture.
In the liquefied petroleum fuel field the term vaporizer is usually employed to designate a heat exchanger in which the liquid is not only converted to a vapor, but is also given some degree of superheat to prevent recondensation downstream in the fuel system. The term evaporator, on the other hand, is usually applied to a heat exchanger which creates vapor that is approximately saturated. This dichotomy in terminology is not precise, and there is some overlap in usage.
A major problem in constructing a compact heat exchanger of either type results from the fact that heat flow per unit area is a function of the temperature difference between the heating and working fluids, and of the time of exposure of the working fluid to the heated surface. In a compact heat exchanger, the residence time of the working fluid is necessarily very short because of the small volume available for the working fluid passages. The problem is more severe in vaporizers, which must not only vaporize the working fluid but add superheat to it.